1. Field of the invention:
This invention relates to a method for the growth of single bulk crystals of compound semiconductors such as ZnS, ZnSe, ZnTe, etc., using the sublimation method or the halogen transportation method, and an apparatus used for the said crystal growth method.
2. Description of the prior art:
The vapor phase transportation method in which crystal materials are transported to a seed crystal by the use of halogen gas is preferably used as a single crystal growth method. According to the vapor phase transportation method, even compounds such as ZnS, etc., having a transition point of about 1020.degree. C. that is below the melting point (about 1830.degree. C.) thereof are grown at a low temperature (e.g., below 1000.degree. C.), so that the passage at the transition point and/or the mixing of polytyped crystals into the single crystal, which may arise in the fusing method in which the compounds are grown at a high temperature (e.g., 1800.degree. C.) under a high pressure (e.g., several tens of atmosphere), can be avoided. Therefore, the vapor phase transportation method in which halogen gas is used as a transporting agent is important to the growth of a single bulk crystal of the above-mentioned compounds. However, since halogen elements (i.e., I, Br, Cl and F) are active even at room temperature (namely, I and Br have a high vapor pressure at room temperature and Cl and F are gaseous at room temperature), a charging process for the charge of a crystal growth vessel with halogen elements becomes complicated. Moreover, it is difficult to accurately add a given amount of halogen to the growth vessel without contamination by foreign substances, while the inside of the growth vessel is maintained at a high vacuum level (e.g., less than 10.sup.-6 Torr) in the charging process. In order to remove these problems, an approach in which the volume of halogen required to provide a given amount after vaporization is measured in advance or an approach in which a given amount of halogen is sealed within a small ampule and then the ampule is added to the growth vessel together with crystal materials has been proposed. The former approach requires measurements of a volume of halogen which necessitate vaporization of halogen using a heating process and solidification of halogen using liquid nitrogen, which causes difficulties in preventing the mixing of water vapor and/or air into the growth vessel in the halogen-charging process. In the latter approach, loss in halogen arises when the halogen-ampule is sealed, resulting in an incorrect amount of halogen. Accordingly, these approaches cannot attain the addition of an accurate amount of halogen with reproducibility to the growth vessel.
As mentioned above, in the vapor phase transportation method in which halogen is used as a transporting agent, prevention of mixing gases such as water vapor, air, etc., into the growth vessel when the growth vessel is charged with halogen is difficult in light of the physical properties of halogen, so that the amount of halogen to be added to the growth vessels cannot be maintained at a fixed value for each vessel, which causes difficulties in establishment of reproduceable crystal growth conditions, resulting in an extremely reduced amount of crystal materials to be transported by the halogen or in the growth of polytype crystals.
In order to solve the above-mentioned problems, a crystal growth method has been proposed in which, as shown in FIG. 3, the temperature distribution is maintained to be in the range of T.sub.1 to T.sub.2 and ampules with a specific design is used, and moreover the seed crystal growth section 23 are separated from the single crystal growth section 25 within the growth vessel 22 so as to regulate the crystal growth. Reference numerals 24 and 26 indicate a crystal material and a heater, respectively. Since this method adopts such a crystal growth environment in which the temperature distribution is in a fixed range of T.sub.1 to T.sub.2 so as to improve the reproducibility of the crystal growth, when the conditions under which halogen is supplied to ampules are different and/or water vapor, air, etc., are mixed into the growth vessel, distorted crystals that are inferior in crystallinity are produced. Moreover, if the growth conditions (including the supply of halogen to ampules, growth temperatures, etc.), are changed in view of the physical property control, there is a possibility that they will vary from the optimal single crystal growth conditions. These problems are caused by the phenomenon that distortions of crystals arising from the beginning crystal growth are taken over by the succeeding crystal growth.